However it is much more than that. Expression of sisu requires also large amount of stubbornness, lack of the instinct of self-preservation, bit of selfishness, small amount of simplicity and, of course when talking about Finns, jealousy.
With this kind of mixture inside of big, blonde guy, Finns are quite hard to beat. Remember for example Finnish Winter war...

(Unfortunately Finns are more and more mixed up with other Europeans and Sisu is diminishing...)